1. Filed of Invention
The present invention relates to a synchronous rectification control, more particularly; relates to an adaptive synchronous rectification control at the secondary side of a transformer for improving efficiency and accuracy.
2. Description of Related Art
An offline power converter includes a power transformer to provide isolation from an AC line voltage to the output of the power converter for safety. In recent development, applying a synchronous rectifier in the secondary side of the power transformer is to achieve a high efficiency conversion for power converters. FIG. 1 shows a conventional power converter with the synchronous rectifier. The conventional power converter comprises a bridge rectifier 10 and a bulk capacitor CIN for converting a power source VAC into an input voltage VIN. The input voltage VIN is stored at the bulk capacitor CIN. A power transformer T1 comprises a primary winding NP in the primary side and a secondary winding NS in the secondary side. The primary side of the power transformer T1 has a power switch Q1 coupled to the primary winding NP for switching the power transformer T1 and for regulating an output voltage VO of the power converter. The power switch Q1 receives a drive signal SG and is coupled between the primary winding NP of the power transformer T1 and a ground.
The secondary winding NS of the power transformer T1 is coupled to the output of the power converter through a synchronous switch Q2 and an output capacitor CO. A drain terminal of the synchronous switch Q2 is coupled to a terminal of the secondary winding NS. A source terminal of the synchronous switch Q2 is coupled to the ground. The output capacitor CO is coupled between the other terminal of the secondary winding NS and the ground. The synchronous switch Q2 and its parasitic diode DQ2 are operated as the synchronous rectifier. Thus, the synchronous switch Q2 having the parasitic diode DQ2 is coupled between the secondary winding NS of the power transformer T1 and the output capacitor CO. The output capacitor CO is coupled to the output voltage VO of the power converter.
A control circuit 20 placed at the secondary side of the power transformer T1 is coupled to a gate terminal of the synchronous switch Q2 for generating a control signal SW at an output terminal OUT of the control circuit 20 to turn on/off the synchronous switch Q2 in response to a detection signal VDET at a detection terminal VDET of the control circuit 20. The detection terminal VDET is coupled to the secondary winding NS. The detection signal VDET is generated at a magnetized voltage VS, a demagnetized voltage and a magnetized period of the power transformer T1. The enabling period of the control signal SW is correlated to the demagnetized period of the power transformer T1. The control circuit 20 includes a comparator 24 and a PWM circuit 25. A positive input of the comparator 24 receives the detection signal VDET. A threshold signal VT is applied with a negative input of the comparator 24. An output of the comparator 24 generates a switching signal SON by comparing the detection signal VDET with the threshold signal VT. The PWM circuit 25 is coupled to the gate terminal of the synchronous switch Q2 for generating the control signal SW in response to the switching signal SON.
FIG. 2A shows the waveforms of the input voltage VIN, the detection signal VDET and the switching signal SON. The input voltage VIN across the bulk capacitor CIN is rectified by the bridge rectifier 10 shown in FIG. 1. The bulk capacitor CIN is served as a voltage regulator, and a ripple range of the input voltage VIN is determined by the capacitance of the bulk capacitor CIN. Thus, the detection signal VDET is changed in response to the ripple range of the input voltage VIN correspondingly. When the threshold signal VT is set too high, the switching signal SON will be missed by comparing the detection signal VDET with the threshold signal VT for a valley voltage of the input voltage VIN. Apparently, for example, the first two lower detection signals VDET are not detected since their amplitudes are lower than the threshold signal VT. Hence, the first drawback of the prior art is that the switching signal SON will be stopped some periods temporarily during the valley voltage of the input voltage VIN once the threshold signal VT is set too high.
FIG. 2B shows the waveforms of the detection signal VDET, the switching signals SON1, SON2) and the drive signal SG disclosed in FIG. 1. FIG. 2B illustrates the detection signal VDET operated in DCM (Discontinuous Conduction Mode). During the normal operation, the switching signal SON is generated in accordance with the comparison between the detection signal VDET and the threshold signal VT. As shown in FIG. 2B, the switching signal SON and the threshold signal VT can be regarded as a first switching signal SON1 and a first threshold signal VT1 respectively. When the threshold signal VT is set too low, an undesirable pulse for the switching signal SON is generated by comparing the detection signal VDET with the threshold signal VT. As shown in FIG. 2B, the switching signal SON and the threshold signal VT can be regarded as a second switching signal SON2 and a second threshold signal VT2 respectively. The second threshold signal VT2 is lower than the first threshold signal VT1. Apparently, for example, the second switching signal SON2 has an additional pulse during a switching period. Hence, the second drawback of the prior art is that the additional pulse in the switching signal SON will be generated for each switching period once the threshold signal VT is set too low.